The present invention relates to a differential device for a four-wheel drive motor vehicle, and more particularly to an arrangement and a structure of a differential device comprising a differential provided between axles and a central differential.
In a four-wheel drive motor vehicle having an engine with a transaxle laterally mounted therein, a central differential is disposed in a transmission case and a differential for one of front and rear wheel drive systems is disposed adjacent the central differential. The power transmission system becomes compact as the central differential is completely housed in a transmission case. However, the construction of the transaxle completely differs from that of a two-wheel drive vehicle. Consequently, the transaxle of the two-wheel drive vehicle can not be used for the four-wheel drive vehicle without greatly changing the structure near the final gear devices. To resolve the problem, there has been proposed a four-wheel drive power transmission system in which the central differential is disposed outside the transmission case of the two-wheel drive vehicle. Therefore, the transaxle of the two-wheel drive vehicle becomes compatible with the four-wheel drive vehicle, thereby enabling one to extensively use the same parts, resulting in a reduction of manufacturing cost.
U.S. Pat. No. 4,693,334 discloses a differential device for the four-wheel drive vehicle having an engine laterally mounted thereof. In the system, the central differential is provided in the transfer case and the front differential is disposed between a transmission case and the transfer case. However, the differential device has a problem as described hereinafter. In the differential device, as a lock-up clutch is disposed besides a ring gear of the central differential, lubricating oil must be the same as the clutch oil, so that separation of the oil is very difficult. Further, the ring gear becomes large because the pinion gears must be inside the ring gear. And the shafts of the pinions are supported by a carrier, so that each pinion is mounted in a hole formed in the differential case and supported in a form of a cantilever and large stress is concentrated in the carrier.
The central differential operates to transmit the output torque of the transmission to the side gears through the differential case, pinion shafts and the pinions. Accordingly, the torque acts to laterally push the pinion shafts, so that the shafts may deflect and sway in accordance with the variation of the torque dependent on the conditions of road surface and engine load. Consequently, the surfaces of the holes in the differential case and the pinion shaft are abraded by excessive surface pressure applied thereto. Particles of the material caused by the abrasion fly into the inner side of the pinions, causing the seizure the pinions to seize, and hence reducing durability and reliability of the system. Moreover, the holes in the differential case are enlarged so that the pinion shafts oscillate, which causes noise.